In metal cutting and grinding processes, in order to increase the lifetime of a tool and improve workability and productivity, a cutting fluid is generally used. Types of cutting fluid for metalworking include a water-soluble cutting fluid and a water-insoluble cutting fluid and the type to be used is determined according to a metal to be processed, process conditions, and required performance. For example, water-soluble cutting fluids are mainly used for dissipating heat generated during high-speed processing and reducing a risk of flammability but have a problem in that the content of base oil, such as mineral oil, included in the composition is small and the lubricity is poor. On the other hand, water-insoluble cutting fluids, the majority of which are composed of base oil, have excellent lubricity due to the action of the base oil, and therefore, are selected when lubricity is required for application to a material that is difficult to cut such as titanium, heat-resistant steel, ceramics, or aluminum alloy and accurate processing. However, commercially available water-insoluble cutting fluids are either fluids which belong to Class IV petroleums under the Japanese Fire Service Act and have a risk of combustion; or fluids which do not belong to Class IV petroleums under the Japanese Fire Service Act but have a high kinetic viscosity at 40° C. of 20 mm2/s or higher and low cooling performance. Therefore, currently, there are no water-insoluble cutting fluids which satisfy the following three conditions: a low kinetic viscosity; high cooling performance; and a high flash point.
In order to obtain a water-insoluble cutting fluid base oil and a water-insoluble grinding fluid base oil which have a low kinetic viscosity and a high flash point, PTL 1 discloses a metalworking fluid composition obtained by mixing an oxygen-containing synthetic oil such as an ester, a mineral oil, and a hydrocarbon oil and adding an extreme pressure agent thereto, in which the kinetic viscosity at 40° C. is less than or equal to 37 mm2/s and the flash point is higher than or equal to 250° C.
However, the kinetic viscosity at 40° C. of the metalworking fluid composition disclosed in PTL 1 is 37 mm2/s, which is not suitable for practical use. For practical use, it is required that the kinetic viscosity at 40° C. be less than or equal to 14 mm2/s. In addition, in PTL 1, a problem related to cooling performance is not solved. In addition, PTL 2 discloses a lubricant base oil in which there is less exhaust smoke and biodegradability and lubricity are improved. However, since the lubricant base oil disclosed in PTL 2 is used for a 2-cycle engine, it is clearly different from an object of the present invention as it is necessary that the lubricant base oil be burned along with fuel after lubrication. Furthermore, regarding the lubrication base oil for a 2-cycle engine, PTL 2 does not disclose the kinetic viscosity, surface tension, and hydroxyl value thereof.
In addition, PTL 3 discloses a base oil which is composed of one or two or more types of mineral oil, fat and oil, and synthetic esters, as one of the components constituting a hot rolling fluid composition. The invention disclosed in PTL 3 relates to a hot rolling fluid composition, which is clearly different from a water-insoluble cutting fluid base oil or grinding fluid base oil according to the present invention. Furthermore, PTL 3 discloses a kinetic viscosity of a mineral oil to be used but does not disclose a kinetic viscosity of an ester, and does not disclose the surface tension and flash point.